The outline for the “Healthy China 2030” emphasizes social media use in health knowledge communication. Health knowledge communication enabled by social media relies heavily on recipients’ willingness to adopt and share health knowledge, which is consequently determined by their trust in content, source, and the platform. However, little is known about the relative importance and differential effects of these trusting beliefs (content credibility, source credibility, and institution-based trust) on health knowledge adoption versus sharing willingness in social media. This study examines this critical issue in the context of WeChat Official Accounts (WOAs) in China. Drawing upon the trust and information communication literature, we develop a research model and test the proposed hypotheses with data collected from an online survey of 2086 WeChat users. The results reveal that health knowledge adoption (sharing) willingness is mainly related to source credibility (institution-based trust). Moreover, content credibility has a stronger relationship with adoption willingness than with sharing willingness, while institution-based trust shows a stronger relationship with sharing willingness than with adoption willingness. These findings extend the literature on trust, information communication, and communication process, and provide rich insights for healthcare institutions to effectively utilize social media (such as WOAs) on health knowledge communication.